Recently, there has been a trend to utilize lasers in many medical applications. For example, some types of laser surgery are performed using carbon dioxide (CO.sub.2) lasers. CO.sub.2 lasers generate a principal wavelength of 10.59 microns which is absorbed in medical treatment predominantly by the water molecules in biological tissue. The absorption and conversion to heat of the laser energy in tissue allows the doctor to cut, cauterize, excise and perform other surgical procedures. CO.sub.2 lasers are also desirable because they can be designed to deliver relatively high power at low cost compared to other types of lasers.
The early CO.sub.2 surgical lasers were of the flowing gas variety. In these CO.sub.2 systems, gas from tanks is continually flowed through the laser tube. Since the early 1980's, sealed gas systems have been available as an alternative to flowing gas systems. While initially higher in cost, the sealed gas systems are less cumbersome to operate and do not require replacement of gas cylinders.
As is well known, another trend in surgery is to localize the intrusion into the body. For example, knee surgery is much less traumatic using arthroscopic procedures. This trend can also be seen where endoscopes have been modified to allow surgical procedures to be performed. More specifically, endoscope were originally designed for viewing internal body parts. The endoscope consists of a cylindrical tube or sheath which can be inserted through a small incision or puncture in the body. It was recognized that if surgical tools could be manufactured to fit within the endoscope, entire surgical procedures could be performed without further invading the body of the patient. Since laser energy can be delivered along a narrow beam, significant effort has been expended to develop laser endoscopic surgical techniques.
One example of the combination of endoscopes and laser surgery is in laparoscopy procedures. In this procedure, an endoscope is inserted through a patient's abdominal wall. Laser light delivered through a laparoscope can be used to burn and remove lesions, adhesions or blockages, for example which might otherwise threaten life, cause pain, or inhibit the patient from becoming pregnant. In any endoscopic procedure using laser energy, it is typical that a purge gas is flowed through the endoscope to prevent smoke and burned tissue from moving up the endoscope and fouling optical elements. In laparoscopies, in addition to the purge gas, the doctor will also typically pump an insufflation gas into the patient's abdomen. The insufflation gas expands the abdomen away from the internal organs to provide a clearer view of the operating field. Where insufflation gas is used, it is even more important to provide a purge gas flow through the endoscope, because the positive pressure in the surgical area would otherwise force burned tissue and smoke into the endoscope, fouling the optics used to focus and deliver the laser beam.
Until a few years ago, a common purge and insufflation gas utilized by surgeons was nitrogen. Unfortunately, nitrogen had many undesirable side effects. For example, nitrogen is not readily flushed from the body and can produce bends-like symptoms in the patient or even disabling or fatal gas emboli in the circulatory system. Accordingly, the recent trend has been to use carbon dioxide (CO.sub.2) as the purge and insufflation gas, because the circulatory and respiratory systems efficiently flush it from the body without complications.
It was recognized by those in the field that when carbon dioxide was used as the purge gas, a certain amount of the power from the CO.sub.2 surgical laser would be absorbed by the purge gas. While room temperature CO.sub.2 has an abundance of molecules in the ground state that will not absorb CO.sub.2 laser light, there will always be a small but definite population of molecules having excited energy states, some of which correspond to the lower lasing level. This small percentage of molecules which populate the lower lasing level will absorb the photons of the CO.sub.2 laser beam at the principal emission wavelength of 10.6 microns.
The absorption of CO.sub.2 laser energy in CO.sub.2 gas has been reported in other situations, particularly in long range communications through the atmosphere. For example, where a CO.sub.2 laser beam is transmitted through the atmosphere over many miles, it will pass through enough atmospheric CO.sub.2 even in low concentration that the absolute number of molecules in higher energy states will be sufficient to attenuate the beam to a significant level. However, this effect was not considered a problem in laser surgery because of the relatively short distances (a matter of inches or, at most, feet) across which the laser beam must traverse the highly concentrated CO.sub.2 purge gas.
The applicant has discovered, however, that the absorption of laser energy by the purge gas can be much more acute than anticipated. In fact, the applicant has discovered that as the laser power is increased, the absorption of laser energy in the purge gas increases nonlinearly, and is caused by the phenomenon of thermal runaway absorption. Thermal runaway absorption has been previously observed in closed gas systems. In such systems, heat which is generated by the absorption of the laser light by the gas cannot be quickly dissipated, and, therefore, tends to heat the gas to a higher level. This heating will increase the population of energetic molecules. When the proportion of energetic molecules increases, the population of molecules in the lower lasing level capable of absorbing the CO.sub.2 laser light increases. This in turn results in greater absorption of energy, greater heating and, again, greater absorption. This rapid cycle is termed "runaway". (See, for example, "Runaway Self-absorption in Multi-kilowatt CO.sub.2 Lasers", Kay and Naylor, Applied Physics Letters, V. 42, No. 3, Apr. 15, 1983).
The problem of thermal runaway absorption is unacceptable in a surgical laser system. As pointed out above, the absorption reduces the power which can be delivered to the patient. The heating of the gas also creates negative lensing which defocuses the beam and hinders proper surgical technique. In addition, the absorption can also heat the delivery apparatus which can cause burns to either the surgeon or the patient.
This effect has been observed by others prior to the invention herein but has been attributed to other factors, such as the optics being misaligned or out of focus. The latter explanation, while inadequate, was convenient since the delivery optics in surgical laser systems required articulated arms with many optical elements that can easily become, and often do, become misaligned. It is now recognized by the applicant that even small misalignments can and do greatly aggravate thermal runaway absorption in the purge gas.
Accordingly, it would be desirable to provide an improved laser system which prevents thermal runaway.
It is another object of the subject invention to provide an improved laser system for maximizing the power deliverable to a surgical site.
It is a further object of the subject invention to provide an improved surgical laser system which minimizes the absorption of the laser energy in the purge and insufflation gases.
It is still another object of the subject invention to provide an improved surgical laser system having a laser wavelength with enhanced absorption in tissue.